At present, an adapter is inserted into a chassis or a panel of some machine (such as a host). To prevent the adapter from falling off, the adapter is usually provided with a reverse plastic buckle, as illustrated in FIG. 1. However, when a large gap exists between the adapter and the chassis or the panel, the adapter is easy to shake. Thus, the adapter and the chassis or the panel need to be secured by screws, consuming time, manpower and material resources. The same problem persists when reverse metal buckles are disposed on two sides of the adapter as illustrated in FIG. 2.